Switching power converters are commonly used to generate regulated power supply rails out of unregulated energy sources. The generated power supply rails are used to power a variety of circuit loads. There are a variety of different types of switching power converters, such as buck converters, boost converters, buck-boost converters, flyback converters, and various other classes of converters. Additionally, switching power converters can be used as signal power amplifiers, such as Class-D amplifiers and switch-mode amplifiers, and also as supply modulators for power amplifiers such as polar power amplifiers. Typically, switching power converters produce output power from an input power source by controlling the relation between the period of time in which a switch, typically a power transistor, is turned “on” and the period of time in which the switch or power transistor is turned “off”. The bursts of energy resulting from this on/off switching process are stored into energy storing elements, i.e. capacitors and inductors, which are then used to provide energy to the load when the power transistor is turned “off”.
By controlling the relation between the “on” and “off” periods of the power transistor using a negative feedback loop and a reference signal (could be a DC reference or an AC signal, or a combination), a voltage at the desired level can be produced at the output. One method to control the relation between the “on” time and “off” time is to keep the total switching period (“on” time plus “off” time) always constant and only change the “on” time. This is referred to as pulse-width-modulation (PWM) control. Another method is to maintain the “on” time always constant, and only change the “off” time. This is referred to as pulse-frequency-modulation (PFM) control. Each of these methods has its advantages and shortcomings, and in many cases a switching converter will incorporate both methods and select between them depending on the application and the use conditions. Regardless of which method is used, the ultimate result is that power can be delivered to the load at the exact levels needed by the load without wasting excess energy across the power transistor. Theoretically, the efficiency of these types of switching power converters can reach 100%, but is typically less due to non-idealities associated with the power transistors and the energy storing elements.
Switching power converters are widely employed in electronic devices due to their high power-conversion efficiency. However, their output, due to periodic switching, typically contains voltage ripples with a frequency spectrum that contains concentrated energy at the switching frequency and its harmonics, i.e., spurs. In many cases, this spurious output noise interferes with the load and significantly degrades its performance. As a result, switching power converters are typically avoided for applications that are sensitive to spurious noise, such as analog and RF applications. In some instances, this spurious behavior can be mitigated using energy-inefficient linear regulators, either directly from the main power source (battery or otherwise), or as a post regulation stage between the noise-sensitive circuitry and the switching power converter. However, this strategy results in much lower power efficiency, as well as increased size and cost. Moreover, as switching frequencies of switching power converters increase, for example into the 3 MHz to 10 MHz range, in order to reduce the size of the passive components needed, and to improve the switching converter's transient performance, linear regulators become less effective in filtering the spurious noise, due to poor power supply rejection at frequencies beyond 1 MHz.
Several techniques for reducing the spurious noise of switching power converters have been proposed over the years. Some take the approach of reducing the peak-to-peak level of the switching noise. Typically, this approach includes using multi-phase regulators, which can significantly increase the cost and implementation complexity, due to the large number of passive components required and the high accuracy of the timing relationship needed between different switching phases. Active ripple cancellation, using different classes of linear amplifiers in parallel with the switching power converter, has also been explored, but this method generally results in poor efficiency due to the high bandwidth required in these amplifiers, as well as the significant ripple current they must provide to cancel out the spurious noise.
Other techniques for reducing the spurious noise of switching power converters that rely on manipulating their switching behavior have also been proposed. One of these techniques includes using either a sigma-delta or a delta modulator in the control loop. While this technique may be able to reduce spurious noise, it may also result in large and broad-band increases in the noise floor, which, in turn, may necessitate the use of additional linear regulators anyway. While the required linear regulators in these cases generally have relaxed power supply rejection requirements, from a power efficiency perspective, this approach is not an improvement over already existing schemes.
It would therefore be desirable to have a system and method of generation spurious noise-free power from a switching power converter. Embodiments of the invention provide such a switching power converter, which is free of spurious noise. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.